Method of field-controlled diffusion and devices formed thereby

ABSTRACT

A technique for creating high quality Schottky barrier devices in doped (e.g., Li + ) crystalline metal oxide (e.g., ZnO) comprises field-controlled diffusion of mobile dopant atoms within the metal oxide crystal lattice. When heated (e.g., above 550 K) in the presence of an electric field (e.g., bias to ground of +/−50 V) the dopant atoms are caused to collect to form an ohmic contact, leaving a depletion region. The size of the depletion region controls the thickness of the Schottky barrier. Metal-semiconductor junction devices such as diodes, photo-diodes, photo-detectors, MESFETs, etc. may thereby be fabricated.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of copending U.S.application Ser. No. 11/615,331, which is incorporated herein byreference and to which priority is claimed.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

This invention was made with government support under AFOSRF49620-02-1-1163 awarded by the Department of the Air Force. Thegovernment has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to metal-semiconductor junctiondevices, and more particularly to devices based on metal-semiconductorjunctions such as Schottky diodes, photo-diodes, MESFETs, etc., producedfrom metal oxides such as ZnO.

2. Description of the Prior Art

Due to its unique material properties, zinc oxide (ZnO) has been andcontinues to be used in optoelectronic components. For example, ZnO is asemiconductor with a direct band gap of 3.37 eV (368 nm at roomtemperature). Its transparency for the visible spectrum and conductivitymean that ZnO can be used as a transparent electrode, for example inoptoelectronic applications such as light emitting diodes (LEDs), laserdiodes, photodiodes, optical displays, etc.

While ZnO may serve as a suitable bulk material for device fabrication,doping is required to actually realize typical semiconductor devices.ZnO can be doped n-type, for example by introduction of Ga or Al as whenthe material is used as transparent conducting oxide. However, reliablep-doping in ZnO has yet to be demonstrated. Consequently, it has notbeen possible to form p-junction or p-n junction devices, such as diodesand diode-based devices like LEDs, laser diodes or photodiodes. Also forelectronic devices such as bipolar transistors and junction FETs pdoping is required.

For certain applications, Schottky diodes (metal-semiconductor junctiondevices) are used as an alternative to semiconductor-semiconductorjunction devices. In addition, Schottky devices enable a number ofunique applications, e.g. MESFETs, Schottky photodiodes, etc. Howeverdue to the low absolute energy of the conduction and valence band ofbulk ZnO, the fabrication of high quality Schottky contacts of ZnO isproblematic. Many unconventional fabrication methods have been proposed,however none of them provides a reliable, reproducible and convenientmethod to fabricate Schottky contacts.

There is little information about ZnO-metal Schottky contacts in theopen literature to date. The chemical reactions between the metal andthe semiconductor, the surface states, the contaminants, the defects inthe surface layer, and the diffusion of the metal into the semiconductorare well known problems in the formation of Schottky contacts. Forinstance, with Al as the contact metal, ZnO produces significantdissociated cations (Zn) in ZnO because of its strong reaction withanions (O) in ZnO. This results in low barrier height and high leakagecurrent.

To create a Schottky barrier with undoped ZnO, a high work functionmetal can be applied to the surface of a ZnO crystal. Although it hasbeen shown that Au presents a number of challenges at high temperatures(>340 K), Au has widely been applied to ZnO to form Schottky barriers.Other metals used for the same purpose are Ag and Pd. It has been foundthat all these reactive metals form relatively high Schottky barriers of0.6-0.84 eV to the n-type ZnO.

Since high-quality Schottky contacts on ZnO are problematic and reliablep-doping of ZnO has not previously been demonstrated, there has beenlittle work addressing the use of ZnO for UV photodetection, includingphotoconductors, Schottky barrier photodetectors,metal-semiconductor-metal (MSM) structures, etc. ZnO photoconductors,consisting of two ohmic Al contacts on N-doped ZnO grown bymetal-organic vapor phase epitaxy (MOVPE) have been reported.(“Ultraviolet detectors based on Epitaxial ZnO films grown by MOCVD,”TMS & IEEE J. Electronic Materials, 27, 69-74 (January 2000),incorporated herein by reference.) At +5 V bias, these devices present adark current of 450 nA, a responsivity of ˜400 A/W and a time responseof 1.5 μs. In a similar material, MSM photodiodes formed withinterdigitated Ag Schottky contacts present lower leakage current (1 nAat 5 V bias) and better spectral selectivity, but slower time response(“ZnO Schottky ultraviolet photodetectors,” J. Crystal Growth, vol. 225,pp. 110-113 (May 2001), which is incorporated herein by reference). Thefast response characteristic of this structure is followed by a slowphotocurrent decay, which lasts for about 5 ms. This slow component isattributed to the oxygen adsorption at the surface and grain boundaries.

In an effort to overcome the difficulty forming p-n junction devices,another approach is to use a second semiconductor material on the p-sideof the device while using ZnO on the n side. Heterojunction diodes havebeen produced in this way, but these devices exhibit substantialmaterial quality issues due to the lattice and thermal mismatch duringgrowth. Nevertheless, efforts continue with regard to studying anddeveloping the photo response properties of these ZnO basedheterojunctions.

For example, Jeong et al. in “Ultraviolet-enhanced photodiode employingn-ZnO/p-Si structure,” Appl. Phys. Lett. 83, 2946 (2003), which isincorporated herein by reference, reported on the photoelectricproperties of a heterostructure n-ZnO/p-Si photodiode which detect UVphotons in the depleted n-ZnO and simultaneously detects visible photonsin the depleted p-Si by employing two related photoelectric mechanisms.The I-V measurements obtained while the photodiodes are exposed toradiation in a wavelength range of 310 to 650 nm showed a linearincrease in photocurrent with reverse bias. In the visible range, thephotocurrent rose rapidly with bias but saturated beyond a criticalvoltage. According to this reference, the diodes exhibited highresponsivities of 0.5 and 0.3 NW for UV (310-nm) and red (650-nm)photons, respectively, under a 30 V bias with a minimum near 380 nmwhich corresponds to the band gap of ZnO.

However, there remain both a desire and a need in the art for techniquesfor producing a high quality Schottky contact device based on ZnO (ormore generally in metal oxides), particularly one with low leakagecurrent and desired optical and I-V properties.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to systems and methodsfor providing high-quality Schottky contacts in metal oxide basematerial system, such as ZnO. A fabrication process and the design ofSchottky-based diodes, photodetectors, metal-semiconductor field effecttransistors (MESFETs), and similar devices on bulk and epitaxial metaloxide is provided.

According to one aspect of the present invention, the electricalproperties of a doped metal oxide base material may be modified by acombination of electrical and thermal conditions leading to migration ofmobile dopant atoms, a process we refer to herein as a “field-controlleddiffusion process.” A dopant atom is understood, for the purpose of thepresent description, as being a deep acceptor/donor or shallowacceptor/donor, which may be added during production of a bulk orepitaxial semiconductor or post-production, and may be addedintentionally or as a part of the semiconductor fabrication process as acontaminant or defect. The fabrication of a Schottky diode is based onthis field-controlled diffusion process. The starting material is ametal oxide such as bulk or epitaxial ZnO into which a dopant (e.g., Li⁺ions) has been incorporated (e.g., by ion implantation or thermallyassisted diffusion process). Depending on the incorporation state (e.g.,interstitial or substitutional at a lattice site) the doping ions act asa donor or deep acceptor. Electric field-controlled diffusion at anelevated temperature allows a controlled movement of the interstitiallyincorporated dopants whereas the substitutional incorporated dopantsremain localized. Depending on the polarity and strength of the electricfield one can create larger or smaller depletion regions underneathcontacts and thereby form and control the thickness of a Schottkybarrier. Accordingly, high-quality Schottky contacts, MOSFETs,Schottky-diodes, etc. with low dark current can be produced.

According to another aspect of the present invention, thefield-controlled diffusion is reversible and can be applied many times.Therefore, a processes is provided for revitalization of devices afterdegradation or damage, or otherwise to adjust device characteristicspost-fabrication.

In the example above, Li provided in ZnO an immobile deep acceptorresulting in a highly resistive material and a mobile shallow donor. Ingeneral, the dopant atoms may but are not required to be amphoteric.That is, the present invention is equally operable should the dopantatoms not form (immobile) deep acceptors. Either way, it is the carrierconcentration being altered by migration of the dopant atoms that iscritical. For example, we could begin with a bulk metal oxidesemiconductor such as ZnO that is highly resistive due to a deep donor,and incorporate a mobile p-dopant to provide mobile acceptors (holes).An electric field may be employed in the semiconductor at an elevatedtemperate environment to accumulate the acceptors in a selected region.The mobile dopant provides holes such that regions where the mobiledopant is dominant are p-doped. In the regions where the material ishighly doped an ohmic contact can be formed, whereas in a region withabsence of the dopant the material is dominated by the deep donor whichenables a Schottky contact. (In these examples it will be appreciatedthat the carrier and dopant types, p- or n-, may be exchanged with theother. That is, a high resistive p-type substrate and mobile n-typedopant.)

These aspects may be obtained by a method in which the device propertiescan be adjusted by a combination of electrical and thermal conditionsleading to migration of mobile dopant atoms.

The above is a summary of a number of the unique aspects, features, andadvantages of the present invention. However, this summary is notexhaustive. Thus, these and other aspects, features, and advantages ofthe present invention will become more apparent from the followingdetailed description and the appended drawings, when considered in lightof the claims provided herein.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings appended hereto like reference numerals denote likeelements between the various drawings. While illustrative, the drawingsare not drawn to scale. In the drawings:

FIG. 1 illustrates a metal oxide structure prior to the process of beingfabricated into a high quality Schottky-barrier device or a high qualityohmic contact according to an embodiment of the present invention.

FIG. 2 illustrates the metal oxide structure of FIG. 1 disposed forfabrication into a high quality Schottky-barrier device or a highquality ohmic contact, below the field-diffusion temperature elevatedtemperature.

FIG. 3 illustrates the metal oxide structure of FIGS. 1 and 2 in theprocess of being fabricated into a high quality Schottky-barrier deviceor a high quality ohmic contact, at the field-diffusion temperature,according to an embodiment of the present invention.

FIG. 4 is a plot of the temperature versus current performance for themetal oxide structure of FIG. 3 in the process of being fabricated intoa high quality Schottky-barrier device or a high quality ohmic contactaccording to an embodiment of the present invention.

FIG. 5 is an illustration of the room-temperature voltage-currentcharacteristics for a device produced by the field-controlled diffusionprocess according to an embodiment of the present invention.

FIG. 6 is an illustration of the room temperature I-V characteristiccurve for the device of FIG. 5 after a field-controlled diffusionprocess resulting in a reversal of the Schottky and ohmic regionsaccording to an embodiment of the present invention.

FIG. 7 is a flow chart of the steps of a general process offield-controlled diffusion according to an embodiment of the presentinvention.

FIG. 8 is an illustration of a Schottky diode fabricated by afield-controlled diffusion process according to an embodiment of thepresent invention.

FIG. 9 is an illustration of the reversal of polarity of the Schottkydiode post-formation by a subsequent field-controlled diffusion processaccording to an embodiment of the present invention.

FIGS. 10 and 11 are illustrations of a Schottky-based photo-detectorfabricated by a field-controlled diffusion process according to anembodiment of the present invention.

FIG. 12 is an illustration of an arrangement for the control of thedepth of photon absorption within a metal oxide structure by use of atransparent semiconducting layer over the metal oxide structureaccording to an embodiment of the present invention.

FIG. 13 is an illustration of a Schottky-based metal-semiconductor fieldeffect transistor (MESFET) fabricated by a field-controlled diffusionprocess according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The field-controlled diffusion process is described below for Li⁺incorporated in ZnO. This description is based on observations of Hallmeasurements on ZnO bulk samples. We have observed that as-formedLi-doped ZnO appears highly resistive. We have concluded that a highlyresistive surface region, caused by ion depletion near the surface ofthe ZnO may be the source of the high resistivity. Our observations canbe explained by making the following assumptions:

-   -   The ZnO bulk crystal has a n-type background doping in the order        of 10¹⁷ cm⁻³    -   The presence of Li is causing acceptor-type (Li—Zn substitution)        and donor-type (Li interstitial) defects. E.g., an acceptor        concentration of 2×10¹⁷ cm⁻³ and a donator concentration of        1.5×10¹⁷ cm⁻³ would result in an average hole concentration of        5×10¹⁶ cm⁻³.    -   There exists a Li-donor (Li interstitial) depletion region close        to the surface. This is causing a high-resistive layer between        evaporated Ti/Au contacts and the low resistive bulk material.        We have developed a technique to overcome the high resistivity        and enable fabrication of ohmic contacts to this material. The        same technique can also be used to control the thickness of the        depletion region and enable high quality Schottky barriers in        this material system.

According to one embodiment of the present invention, a high voltage(e.g., +/−50-100V) is applied between two points on a structure ofcrystalline ZnO at an elevated temperature (>550 K). The electric fieldmay be established between two contacts (e.g., 5 mm apart) on thesurface of the metal oxide body, the body may be placed in an externalfield or by other means. A process of selectively diffusing interstitialLi atoms (Li has a lower diffusion barrier than LiZn) is thus initiated.The positively charged Li interstitials are attracted by the grounded ornegatively biased contact. This destroys the depletion region andcreates an ohmic region at that contact. At the positively chargedcontact the Li ions are repelled, causing a larger depletion region andtherefore a thicker Schottky barrier. We have also determined that thisprocedure is reversible. The benefits of this process and itsreversibility are discussed further below.

With reference to FIG. 1, a device 10 in the process of being fabricatedinto a high quality Schottky-barrier device or a high quality ohmiccontact is illustrated. Device 10 begins with a crystalline metal oxidematerial 12, either as a bulk substrate or epitaxially formed on asuitable substrate (not shown). ZnO is a desirable initial metal oxidematerial due to its band gap, conductivity, and optical properties.Dopant atoms 14 a, 14 b are introduced into metal oxide material 12. Thedesired properties of at least one of the dopant atoms 14 a, 14 binclude low molecular weight and small size in order to permit mobilitywithin the metal oxide crystalline structure. They must be either n-typeor p-type ions. With these attributes in mind, we have identified Li⁺ asa good candidate material for dopant atoms 14 a, 14 b. Dopant atoms 14a, 14 b may be introduced during formation or growth of metal oxidematerial 12, or subsequently introduced by implantation, thermaldiffusion or other technique. In the process of introduction, dopantatoms may be interstitial, 14 a, or substitutional 14 b within the metaloxide material crystal structure. In this way, there are two types ofdopants present: mobile, such as interstitial dopant atoms 14 a, andfixed/immobile, such as substitutional dopant atoms 14 b. In itsas-formed (or as-doped) state, metal oxide material 12 will have dopantatoms 14 a, 14 b generally randomly distributed throughout. In general,dopants 14 a, 14 b do not have to be amphoteric, interstitial orsubstitutional. Rather, what is required is that one dopant (14 b) iscausing an immobile deep donor or deep acceptor such that the materialappears high resistive and a second dopant (14 a) that is providing amobile shallow acceptor or shallow donor, respectively.

As illustrated in FIG. 1 the upper region (layer) 13 close to surface ofoxide material 12 is depleted of the dopant atoms (14 a) with the highermobility, as compared to the region 15. This is a typical staringcondition of a doped oxide material structure due to a temperature stepduring the fabrication of the oxide material and/or doping, causing anout-diffusion of atoms 14 a. Note that the dopants 14 b are Li on Znsites (deep acceptors, producing high resistivity). In the bulk material12 the deep acceptors 14 b are overcompensated by shallow donors 14 a(the interstitials) causing a net n-type background.

In one example of the above process, ZnO bulk material was employed. Lidopant atoms were incorporated after the growth of the ZnO by annealingof the material in a Li-bearing atmosphere (LiOH). Annealingtemperatures were in the range of 500° C. and 900° C. The Ti/Au contactswere thermally evaporated onto the sample through a shadow maskaccording to procedures well known in the art. Contact thicknesses were20 nm of Ti and 100-200 nm of Au.

Device 10 at this stage will exhibit a fairly uniform, relatively highelectrical resistivity. The material has a homogeneous bulk n-typebackground doping but the bulk properties are difficult to measure sincethey are screened by the high resistive surface layer 13. Due to thehigh resistive surface layer 13 it is difficult to create ohmic contactson the bulk material. We have also found, however, that the electricalproperties of the bulk material may be modified by a combination ofelectrical and thermal conditions leading to migration of the mobileinterstitial dopant atoms 14 a, a process we refer to as a“field-controlled diffusion process”.

According to our field-controlled diffusion process, if the temperatureof metal oxide material 12 is raised sufficiently high, and asufficiently strong electric field is established between two points onmetal oxide material 12 at the elevated temperature, Schottky-like I-Vcharacteristics are obtained. It appears that, at the elevatedtemperature and in the presence of the electric field, the dopant atomstend to migrate toward one of the field points creating there an Ohmiccontact and revealing the Schottky contact at the other field pointwhere the mobile atoms migrate away from the contact (and so increasingthe Schottky depletion region).

For example, according to one embodiment of the present invention shownin FIG. 2, first and second contacts 16, 18 are formed approximately 5mm apart on a surface of metal oxide material 12. When a voltage bias of−50 volts is applied to one of the contacts, for example second contact18, at room temperature (without any pre-treatment of the device) poorcurrent across the contacts is obtained. However, when the arrangementis repeated while the device 10 is at an elevated temperature of ˜550 K,we note a migration of dopant atoms 14 a toward second contact 18, asillustrated in FIG. 3. When the voltage bias is removed and the device10 returns to room temperature, dopant atoms 14 a remain in thelocations to which they migrated. The positively charged interstitialdopant atoms are attracted by the negatively biased contact. Thisdestroys the depletion region immediately thereunder, and creates anohmic contact. At the ground contact the dopant atoms are repelled,causing a larger depletion region, creating a thicker Schottky barrierthereunder.

We also note that when a voltage bias of +50 is applied to secondcontact 18 while the device 10 is at a temperature at or aboveapproximately 550 K, the polarity of the Schottky device reverses, dueto migration of the dopant atoms away from second contact 18 and towardfirst contact 16. An ohmic region is formed under first contact 16, anda Schottky barrier is formed under second contact 18. This attribute ofour process means that device characteristics may be controlled, andeven reversed, post-fabrication. Modifying the characteristics of thedevice may, in fact, be done many times. For example, thefield-controlled diffusion processes may be employed for revitalizationof devices after degradation or (radiation) damage.

With reference to FIG. 4, the field-controlled diffusion process forcreating an Ohmic contact is illustrated as a function of temperatureversus current. Note that the current through this contact is initially(before the field-controlled diffusion process) in the 1 to 10 nA range,and rises to 0.1 to 1 mA after the diffusion process at a voltage of 50V. The increased Schottky depletion region underneath the other contactis causing low reverse currents (discussed further with regard to FIG.5, below). It will be seen that as the temperature rises, the currentbetween contacts increases (region 22), and after a point, approximately550 K in this example, the device becomes significantly more conductive(region 24) and far less dependent upon temperature. Note that the“kink” around 550 K in the 1^(st) cycle curve indicates the onset of thefield-controlled diffusion. In the example shown in FIG. 4, a secondfield-controlled diffusion process was performed on the sample, andagain this effect was seen (region 26), although with much less dramaticeffect than the first processing. This indicates that the field induceddiffusion process was not completed after the first temperature cycle.Further field-controlled diffusion steps did not significantly alter thedevice conductivity, indicating a significantly complete migration ofthe mobile dopant atoms.

FIG. 5 is an illustration of the room-temperature voltage-currentcharacteristics for a device produced by the field-controlled diffusionprocess according to the present invention. As can be seen, the deviceperforms as would be expected of a forward and reverse biased Schottkydevice. Likewise, FIG. 6 is an illustration of the room temperature I-Vcharacteristic curve for the device of FIG. 5 after a field-controlleddiffusion process resulting in a reversal of the Schottky and ohmicregions. That is, the device whose performance is illustrated in FIG. 6is reversely biased compared to that of FIG. 5. Again, the deviceperforms as would be expected of a forward or reverse biased Schottkydevice, which demonstrates the reversibility of the field-controlleddiffusion process. Note that the poor performance of the Schottkycharacteristics in forward bias is mainly caused by the seriesresistance between the two test contacts which were 5 mm apart.

Steps of the general process 40 of field-controlled diffusion accordingto the present invention are shown and described with reference to theflow chart of FIG. 7. A metal oxide body is prepared, possibly includinggrowth thereof, at step 42. Dopant atoms are introduced at step 44,either during formation of the body at step 42 (i.e., formation anddoping done in a single step, hence the dashed line between steps 42 and44) or subsequent to forming the body (e.g., by diffusion, implantation,etc.) At least two contacts are next formed on body, at step 46. Thestructure so formed is then heated at step 48. While the appropriatetemperature to permit dopant atom mobility will depend on the choice ofmetal oxide material forming the body as well as the choice of dopantatom material, we have found that for ZnO and Li dopants, heating thestructure to at least 550 K is sufficient. At step 50, an electric fieldis created between the at least two contacts. While the appropriatefield strength to cause migration of the dopant atoms will depend on thechoice of metal oxide material forming the body as well as the choice ofdopant atom material, we have found that a field formed between a firstcontact at ground and a second contact at +/−50 volts is sufficient. (Itwill be noted that the order of steps 48 and 50 may be reversed withoutchanging the result of the process.) Finally, the electric field isremoved and the device returned to room temperature at step 52 toeffectively lock the migrated dopant atoms into place. Additional stepsrequired to form specific devices may then be performed at step 54, asdescribed further below (noting that returning the device to thefield-controlled diffusion temperature and electric field conditions mayalter the Schottky barrier(s) established in the preceding steps). Itwill be appreciated that some or all of the processing underlying step54 may precede steps 48 through 52 in appropriate applications.

The foregoing describes the generalized formation and operation of aSchottky barrier within a metal oxide material. While according to thepresent disclosure, ZnO is a system of particular interest, a Schottkybarrier may be formed by field-controlled diffusion in a variety ofmetal oxide material systems. Furthermore, while the foregoing hasdescribed Li as a suitable dopant material within a ZnO base material,other dopant materials may be employed, as will be appreciated by oneskilled in the art. Finally, devices according to the present inventiondescribed below include Schottky diodes, photodetectors, MESFETs, etc.,many other devices employing Schottky barriers may benefit fromformation according to the present invention, and accordingly thepresent invention shall not be interpreted as being limited to thefabrication of the devices explicitly discussed below.

The field-controlled diffusion process of the present invention enablesthe formation of a number of useful devices. A first such device 60, aSchottky diode, is illustrated in FIG. 8. As previously described, ametal oxide body 12, such as ZnO is formed to include dopant atoms 14 aand 14 b, such as Li interstitials (n-type dopant) and Li on Zn site(deep p-type) therein, and a field-controlled diffusion process isperformed, resulting in a relatively larger population of n-type dopantatoms under contact 18, which creates an ohmic contact, and a relativelylarger depletion region under contact 16 which creates a relativelythicker metal-semiconductor Schottky barrier. Accordingly, betweenterminals A and B a Schottky diode 62 is effectively formed. Of course,the contacts can be applied at any side of the crystal, e.g. the ohmiccontact could be formed on the backside of the bulk or epi crystal.

As mentioned, care must be taken that the operating conditions of thedevice formed are not at or above the field-controlled diffusionconditions used to form the ohmic contact and depletion regions above.

However, in certain circumstances, it may be desirable to refresh (e.g.,repair damage), alter (e.g., tune performance parameters), or entirelyswitch (e.g., change polarity) performance aspects of the devicepost-formation. For example, the field-controlled diffusion process maybe repeated following formation of the device shown in FIG. 8 toreestablish the depletion and ohmic contact regions which form theSchottky diode 62. Furthermore, as illustrated in FIG. 9, the polarityof the Schottky diode may be reversed post-formation by a subsequentfield-controlled diffusion process. Beginning with the device shown inFIG. 8, a voltage of −50 V is applied to terminal B while terminal A isconnected to ground. (Alternatively, a voltage of +50 V is applied toterminal A, while terminal B is connected to ground, or a voltage of +50V is applied to terminal A, while a voltage of −50V is applied toterminal B.) The temperature of the device is raised above thefield-controlled diffusion temperature (e.g., above 550 K). Assumingp-type dopant atoms are employed, those dopant atoms are therebyaccumulated in the region of first contact 16 forming an ohmic contact,and a depletion region is formed under second contact 18, effectivelyforming a Schottky diode 66 having a polarity revered as compared tothat of diode 62 shown in FIG. 8. This reversal of the ohmic andSchottky contacts is itself reversible, and the process can be repeatedmultiple times for a given device.

A second device, a Schottky-based photo-detector 70, is illustrated inFIGS. 10 and 11. According to an aspect of the present invention, Ohmiccontact 74 is formed on the backside of ZnO body 72. Body 72 has mobilen-type dopants 76 a and immobile deep acceptors 76 b therein. An array80 of finger contacts is formed on the surface of body 72 oppositecontact 74. Initially, a field-controlled diffusion process is performedsuch that a bias, such as −50 V, is applied to array 80, while contact74 is, for example, connected to ground (or, as with any of theembodiments herein, connected to a positive potential such as +50 V).Device 70 is raised to above about 550 K, causing the n-type Li⁺ dopantatoms to migrate further toward ground and contact 74. With thetemperature returned to room temperature, and the bias removed fromarray 80, a device is formed having a depletion region proximate array80, and an Ohmic region proximate contact 74. A Schottky barrier is thusdeveloped between array 80 and contact 74. When exposed to light,photons are absorbed within the ZnO body 72 (for ZnO, absorption will bein the UV and blue wavelengths) creating electron-hole pairs. Theelectrical field within the depletion region of the Schottky contact canseparate the electron-hole pairs and cause a photocurrent through theSchottky contact. A photodiode may thus be formed. By tuning thethickness of the (Schottky) depletion region under array 80 aphotodetector with a controlled space charge region and optimizedcapacitance can be designed.

The array of finger contacts 80 serve to form one connection to thephotodiode so formed. The gaps between the contacts 80 permit light tobe incident on the body 72 when the contacts 80 themselves are opaque tothe wavelength of interest. However, the finger contact may be formed ofa transparent conductor, such as ITO, or may be replaced by atransparent layer of conductive material, again such as ITO or asemitransparent thin metal film. These selections depend on theapplication of the photodetector.

The thickness of the Schottky depletion region defines the absorptionvolume. The thickness of the depletion region can be altered by applyinga voltage and it is influenced by the carrier concentration within thesemiconductor. Further, the relative position of the work function ofthe metal compared to the Fermi level in the semiconductor influencesthe thickness of the depletion region. E.g. for a carrier concentrationof 10¹⁶ cm⁻³ the depletion region is typically in the order of 1 micron.If the carrier concentration is increased the depletion region sizedecreases. Finally, if the depletion region becomes very thin, electronscan tunnel through the energetic barrier between metal and semiconductorbulk such that Schottky contacts become Ohmic.

In some applications it may be desirable to control the depth of photonabsorption within ZnO body 72. One method of accomplishing this is toform a non-absorptive layer 82, which is transparent within bandwidth ofinterest, between array 80 and body 72, such as illustrated in FIG. 12.Non-absorptive layer 82 is created on the surface of the structurethereby eliminating (or at least significantly reducing) carrierrecombination at the surface of body 72 (a well known loss mechanism ofphotodetectors) that would not contribute to the photocurrent.Non-absorptive layer 82 is typically a semiconductor that has a largerband-gap than the underlying material forming body 72. One example wouldbe MgZnO which has a larger band-gap than ZnO. The depletion region of aSchottky contact (where the charge separation of photo-generatedelectron-hole pairs takes place) is typically around 1 or 2 microns deepdepending on the carrier concentration within the semiconductor. Thelarge-band-gap semiconductor layer should be comparatively thin, e.g.,10-100 nm, as it reduces the effective absorption volumes.

A third device based on the present invention, a Schottky-basedmetal-semiconductor field effect transistor (MESFET) 90 is illustratedin FIG. 13. According to this aspect of the present invention, a ZnObody 92 is formed to include Li⁺ dopant atoms (interstitial andsubstitutional) 94 a, 94 b therein. Three contacts 96, 98, 100respectively, are formed on the surface of body 92. Body 92 is formed ona highly resistive substrate 102 opposite contacts 96, 98, 100 whichdoes not allow currents to run from contact 96 to contact 100.Field-controlled diffusion is performed such that the Li+ dopant ionsare attracted to the regions below contacts 96 and 100. Dopant atomsunder contact 98 may be in their as-processed state (generally adepletion region directly under contact 98), or contact 98 may beconnected to a potential such that a field is created causing dopantatoms to migrate away from the region below contact 98. In this way, theregion immediately below contacts 96 and 100 form Ohmic contacts and theregion immediately below contact 98 forms a Schottky contact.

In essence, this process produces a structure in which contact 96 mayserve as a source, contact 98 may serve as a gate, and contact 100 mayserve as a drain for a MESFET. If the depletion region below the gate(98) is small, a current can run from source (96) to drain (98) alongthe highly conductive regions in which the mobile Li+ donor atomsaccumulate. If a voltage is applied to the gate, the depletion regionmay be made to extend the entire depth of body 92 under the gate (98)such that no current can run from source (96) to drain (100).

While a plurality of preferred exemplary embodiments have been presentedin the foregoing detailed description, it should be understood that avast number of variations exist, and these preferred exemplaryembodiments are merely representative examples, and are not intended tolimit the scope, applicability or configuration of the invention in anyway. Rather, the foregoing detailed description provides those ofordinary skill in the art with a convenient guide for implementation ofthe invention, and contemplates that various changes in the functionsand arrangements of the described embodiments may be made withoutdeparting from the spirit and scope of the invention defined by theclaims thereto.

1. A method of fabricating a metal-semiconductor Schottky-barriertransistor structure, comprising: providing a metal oxide structurehaving mobile p-type dopant ions introduced therein; forming first,second, and third contact structures on the surface of said metal oxidestructure wherein the second contact is located physically between, butspaced apart from, the first and third contact structures; elevating thetemperature of said metal oxide structure; creating an electric fieldwithin a first region and a third region of said metal oxide structureby establishing a voltage potential between the first and second contactstructures and the second and third contact structures, respectively,said electric field present while said metal oxide structure is at saidelevated temperature, such that said mobile p-type dopant ions arecaused to selectively migrate from a second region of said metal oxidestructure which is substantially outside of said electric field towardsaid first and third regions of said metal oxide structure proximatesaid first and third contact structure; and removing said electric fieldand lowering the temperature of the metal oxide structure such that themigrated dopant ions remain in the locations to which they havemigrated; whereby at least a portion of said second region becomes oneportion of a Schottky barrier, and further whereby exposure of thestructure to light in order to cause the mobile p-type dopant atoms tomigrate is not performed.
 2. The method of claim 1 wherein the step ofcreating an electric field in the metal oxide structure while thestructure is at an elevated temperature results in the formation at theinterface between the contact structures and the metal oxide structureat the first and third regions Ohmic contacts, and at the second regiona Schottky contact.
 3. The method of claim 1 wherein the first and thirdregion provides a source region and a drain region of the transistor,respectively, and the second region provides a gate region for thetransistor.